1 . Field of the Invention
This invention relates generally to storage and dispensing systems for the selective dispensing of fluids from a vessel in which the fluid component(s) are sorptively retained by a sorbent medium, and are desorptively released from the sorbent medium in the dispensing operation. More specifically, the invention relates to such type of storage and dispensing system from which the sorbate fluid is supplied by flow of carrier gas through the storage and dispensing system during the dispensing operation.
2. Description of the Related Art
In a wide variety of industrial processes and applications, there is a need for a reliable source of process fluid(s), which is compact, portable, and available to supply the fluid(s) on demand. Such processes and applications include semiconductor manufacturing, ion implantation, manufacture of flat panel displays, medical treatment, water treatment, emergency breathing systems, welding operations, space-based applications involving dispensing of liquids and gases, etc.
U.S. Pat. No. 4,744,221 issued May 17, 1988 to Karl O. Knollmueller discloses a method of storing and subsequently delivering arsine, by contacting arsine at a temperature in the range of from about -30.degree. C. to about .+-.30.degree. C. with a zeolite of pore size in the range of from about 5 to about 15 Angstroms to adsorb arsine on the zeolite, and then dispensing the arsine by heating the zeolite to an elevated temperature of up to about 175.degree. C. for sufficient time to release the arsine from the zeolite material. Knollmueller utilizes thermal desorption to effect dispensing of the arsine at elevated pressures. An optical sensor or spectrophotometer is described in the patent as being used to monitor arsine concentrations. The Knollmueller system operates at pressures that are &gt;15 psig and at temperatures in excess of 60.degree. C. to dispense arsine at concentrations of 15-60% by volume.
U.S. Pat. No. 5,518,528 issued May 21, 1996 in the names of Glenn M. Tom and James V. McManus, describes a gas storage and dispensing system, for the storage and dispensing of gases, e.g., hydride gases, halide gases, organometallic Group V compounds, etc., which overcomes various disadvantages of the gas supply process disclosed in the Knollmueller patent.
The gas storage and dispensing system of the TQm et al. patent comprises an adsorption-desorption apparatus, for storage and dispensing of gases, including a storage and dispensing vessel holding a solid-phase physical sorbent medium, and arranged for selectively flowing gas into and out of the vessel. A sorbate gas is physically adsorbed on the sorbent medium. A dispensing assembly is coupled in gas flow communication with the storage and dispensing vessel, and provides, exteriorly of the vessel, a pressure below the vessel's interior pressure, to effect desorption of sorbate gas from the solid-phase physical sorbent medium, and flow of desorbed gas through the dispensing assembly. Heating means may be employed to augment the desorption process, but it is preferred to operate the Tom et al. system with the desorption being carried out at least partially by pressure differential-mediated release of the gas from the sorbent medium.
The storage and dispensing system of the Tom et al. patent embodies a substantial advance in the art, relative to the prior art use of high pressure gas cylinders. Conventional high pressure gas cylinders are susceptible to leakage from damaged or malfunctioning regulator assemblies, as well as to rupture and unwanted bulk release of gas from the cylinder if the internal gas pressure in the cylinder exceeds permissible limits. Such over-pressure may for example derive from internal decomposition of the gas leading to rapid increasing interior gas pressure in the cylinder.
The fluid storage and dispensing system of the Tom et al. patent thus reduces the pressure of stored sorbate gases by reversibly adsorbing them onto a sorbent medium, e.g., a zeolite or activated carbon material.
A fluid storage and dispensing system of the general type disclosed in the Tom et al. patent, hereafter referred to as an "FSDS," is commonly used in association with low pressure end use applications, such as ion imnplantation in semiconductor manufacturing processes, that provide a pressure differential, .DELTA.P, between the near-ambient interior pressure of the storage and dispensing vessel, P.sub.FSDS, and the low pressure of the downstream process in which the dispensed fluid is used, P.sub.use. The near-ambient pressure of the FSDS vessel's interior volume may for example be on the order of 600 to 800 torr, and the pressure of the downstream process may for example be below 500 torr.
In these low pressure applications, the pressure differential, .DELTA.P=P.sub.FSDS -P.sub.use, is favorable for extracting the sorbate fluid from the FSDS vessel, including the fluid in the head space and in the interstices of the bed of sorbent material (typically present as a bed of particulate, pelletized, bead, granular or other finely divided material having affinity for the sorbate gas species of interest), as well as effecting desorption of the sorbate fluid from the sorbent material by virtue of the pressure differential.
Difficulties arise, however, in dispensing fluid from an FSDS when the pressure level of the downstream process is at atmospheric or slightly below atmospheric pressure, and the pressure differential, .DELTA.P=P.sub.FSDS -P.sub.use, is very low. In many instances, this pressure differential driving force (for mass transfer of the sorbate gas from the sorbent material into the surrounding gas phase of lower concentration of the sorbate species) is by itself inadequate to achieve the desired rate of dispensing of the sorbate gas.
Under such atmospheric or near-atmospheric dispensing conditions, therefore, there exists little or no driving force for mass transfer, i.e., there is little or no pressure gradient for achieving release of the sorbate fluid from the sorbent medium. As a result, only low or even minimal mass transfer of the sorbate gas may be possible. This is true for sub-atmospheric pressure processes, such as chemical vapor deposition (CVD) of tetraethylorthosilocate (TEOS), and is especially true in applications such as silicon epitaxy and other processes operating at atmospheric pressure.
An alternative means of achieving the desired desorption is the use of heat inputted to the sorbent material to mediate thermal desorption. Heating of the sorbent material in the storage and dispensing vessel of the FSDS shifts the equilibrium isotherm of the sorbent material so that the sorptive capacity of the sorbent material for the sorbate fluid is reduced. This in turn facilitates the desorption of the sorbate fluid from the sorbent material as the sorbent material is heated.
At the same time, however, the sorbent material has a finite heat capacity, and this "thermal mass" prevents the sorbent material from being instantly cooled when the input of heat is terminated at the fmish of the dispensing operation. As a result, even though the dispensing of the sorbate fluid has been concluded, fluid continues to desorb from the hot sorbent material and remain in the desorbed state, causing an overpressure in the FSDS vessel. Such pressure increase is at odds with the desired low pressure condition for the FSDS vessel interior volume, and the pressure increase may with significant heating create a hazardous high pressure level in the FSDS vessel.
It is an object of the present invention to provide an improved fluid storage and dispensing apparatus and method which overcome the aforementioned difficulties of the prior art.
It is another object of the present invention to provide an improved fluid storage and delivery system which utilizes little or no heating to supply the sorbate fluid for downstream applications which are at atmospheric pressure or above-atmospheric pressure levels.
It is a further object of the present invention to provide an improved means of controlling the dispensing flow rate and concentration of sorbatel fluid from a fluid storage and dispensing system.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.